Predicting the Heading Misalignment of a Vessel-Shaped Offshore Fish Farm Under Waves and Currents

Abstract

A vessel-shaped fish farm concept for open sea applications has been proposed recently. The whole system consists of a vessel-shaped floater, fish cages positioned longitudinally along the floater, and a single-point mooring system. The whole system weathervanes; this feature increases the spread area for the fish waste. However, the downstream cages may experience reduced water exchange when the vessel is parallel to the currents. This situation may jeopardize the fish health. A dynamic positioning (DP) system may be necessary to improve the flow conditions. This paper investigates the misalignment angle between the heading of the vessel-shaped fish farm and the currents under combined wave and current conditions. The misalignment angle is critical for the estimation of the DP system consumption. A numerical model of the fish farm system with flexible nets is developed. Current reduction factors are included to account for the flow velocity reductions between the net panels. The heading of the system is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. An integrated method using metamodels is proposed and applied for the prediction of the misalignment angle for a reference site. The probability distribution of the misalignment angle between the vessel heading and the currents is calculated using the Kriging metamodel for the reference site. Based on the prediction, the requirement for the DP system to improve the flow condition in the fish cages is discussed.